Troubleshooting Brake Controller Installations
Common Brake Control Issues
Problem
Possible Issues
Testing Procedures
Short on brake signal wire, trailer or vehicle side. Corrosion in trailer and/or vehicle plug. Short on brake magnets on trailer. Malfunctioning brake magnet(s).
Method 1 Method 2 Method 3 Method 4
Ground wire was not connected to negative battery terminal, ground area does not have solid connection to ground. Adapter wiring does not match OEM connector on vehicle. Ground connection to negative battery terminal is loose. Ground wire has been damaged and exposed wiring is touching a metal area.
Connect ground wire to negative battery terminal. Method 1 Method 3
Brake assemblies are not properly adjusted. Boost/Gain on brake controller is set too high for trailer size.
Adjust brakes properly (see video). See brake control owner's manual for proper boost/gain settings for trailer size.
Incorrect stoplight wire connection. Adapter wiring layout does not match OEM connector on vehicle.
Method 1 Method 2
Trailer has too many brakes for brake controller to handle. Corrosion in trailer and/or vehicle plug. Brake signal wire on trailer is damaged, exposed wiring is touching metal area or another circuit. Malfunctioning brake magnet(s).
Check brake control owner's manual for limit on number of trailer brakes. Method 1 Method 3 Method 4
Short to battery on 12V wire connection. Short on brake signal wire, trailer or vehicle side Corrosion in trailer and/or vehicle plug. 12V wire was not connected to vehicle battery, 12V source used is not constant.
Method 2 Method 3 Connect 12V wire to positive battery terminal.
Corrosion in trailer and/or vehicle plug. Brake signal wire on trailer or vehicle is damaged, exposed wiring is touching metal area.
Method 3
Incorrect stoplight wire connection. Corrosion in trailer and/or vehicle plug. Adapter wiring does not match OEM connector on vehicle.
Method 1 Method 3
Connection to battery is not complete. Vehicle is missing a fuse or relay required to power 12V power circuit. Adapter wiring layout does not match OEM connector on vehicle.
Method 1 Check fuse block for missing fuse/relay. See vehicle-specific FAQ if available, or check owner's manual.
Basic Testing Procedures
Method 1: Verifying Correct Wire Connections (Hard Wire Installs)
Make sure that each wire is connected in the correct location. Standard wire colors are listed, but check connections by function if colors vary. 
Verify that the white brake controller wire (ground function) is connected to the negative battery terminal. Verify that the black brake controller wire (12V power function) is connected to the positive battery terminal with a 20 or 30 amp circuit breaker installed in line. For trailers with 2-4 brakes, use a 20 amp breaker and for trailers with 6-8 brakes, use a 30 amp breaker. Use a 12V probe-style circuit tester to verify the stoplight wire (red) connection (Figure 1). Click here to see our circuit testers. The tester should show power when the brake pedal is pressed, and should not show power when the pedal is released. If all connections are correct, proceed to Method 2.
Method 1: Verifying Correct Wire Connections (Plug-in Installs)
Use a 12V probe-style circuit tester to verify that power is present at OEM connector. You should be able to get a solid power reading on one pin/slot, a reading on a second pin/slot only when the brake pedal is pressed, and at least two other pins with no power present. Verify that the pin/slot layout of the wiring adapter you are using matches the layout in the OEM connector. The two plugs should be mirror images. Check the part number on the adapter and make sure it is the same as the listed fit for your vehicle. If all connections are correct, proceed to Method 2.
Method 2: Testing for the Brake Signal
Use a 12V probe-style circuit tester to verify that power is present on the blue wire behind the brake controller when you use the manual control (Figure 2). If no power is present, the brake controller may be malfunctioning and require replacement. See video for reference. Without the trailer plugged in, use a circuit tester to verify that power is present at the vehicle 7-Way when the brake pedal is pressed or the manual control is used. The electric brake pin should be in the 5 o'clock position. If no power is present, proceed to Method 3. With the trailer connected, check for power on the electric brake wire coming out of the trailer's 7-Way plug. You can repeat the test on this wire, moving further back toward the magnets each time. If power stops, look for a short or ground issue (Method 3).
Method 3: Looking for a Short or Ground Issue
If controller was hard wired (controller wires were spliced - no plug used to connect to OEM brake controller wiring), verify that the white ground wire is attached to the negative battery terminal on the vehicle. Check along any wiring that was run from front to back on the vehicle and look for spots that have been nicked, pinched, or rubbed and have copper wire showing through the jacketing. Wrap any damaged areas with electrical tape. Inspect the trailer wiring that runs from the trailer plug, again looking for any damaged spots (Figure 3). Wrap any damaged areas with electrical tape or replace the wire section, if the damage is great enough. Look inside the front and back sides of the vehicle and trailer plugs to determine if any white or green corrosion has accumulated over time. If you see any corrosion, you will need to either thoroughly clean the plug or replace it. A wire brush works best for cleaning. Check the wires on the trailer brake magnets. Each magnet should have two wires: one for power, and one for ground. Make sure that the ground wire is attached to a clean metal area without any paint or corrosion. The power wire should be intact and have a solid connection to the electric brake signal wire. It does not matter which wire does what, just that one wire is powered and the other is grounded.
Method 4: Inspecting the Brake Assemblies
Visually inspect brake magnets for excessive wear. If the magnets have heavy grooving, are worn unevenly, or the internal windings are visible, replace the magnet. See Trailer Hub Brake Magnet Replacement Demonstration for reference. If magnets are not visibly worn, refer to Testing Trailer Brake Magnets for Proper Function. Test each magnet for general function, for a short, and for proper resistance. You will need a 12V battery and a multimeter to complete the testing. Ensure that the brake assemblies are properly adjusted. The brake shoes should be adjusted out until you cannot turn the wheel by hand. You would then back off the adjuster 8-10 clicks, or until you feel the wheel has a slight drag as you turn it by hand. Refer to This Old Trailer: Electric Trailer Brake Installation Part 1.
For more information, see:
Vehicle-specific FAQ articles, if available. Trailer Brake Controller Information Brake Controller Installation: Starting from Scratch Brake Controller 7- and 4-Way Installation Kit (ETBC7)
Kola
5/3/2025
I just got a brand new Primus IQ. Plugged it in and immediately it got super hot, way too hot to touch. The wiring harness was also fairly hot. Any ideas what the heck could be wrong with it?
Robin H.
5/13/2025
@Kola is this on a Nissan? It looks like you got a Nissan harness. There have been some issues with certain years/models where they swap the factory wires around so that could be the problem. I would test the wires on the vehicle side and make sure they are correct, specifically the 12V power wire. If they aren't matched up correctly that would explain the issue and you would have to re-pin one of the connectors.
Phiib
3/20/2025
I have Prodigy P2 brake controller, which recently began displaying the P.L code. I’m not clear how to troubleshoot the power interruption issue. Is this internal to the controller? The seven-pin plug? The electrical connection to one of the brakes?
Andrew N.
3/20/2025
@Phiib Hey there, sorry about the issue you're having. That just means there is an issue in the wiring somewhere on the power wire (black wire) going to the controller. I'd check to make sure all connections are tight and free of corrosion from the battery to the controller. There also should be a circuit breaker inline on that black wire, and sometimes if that is failing, it can cause this issue. You can try to temporarily bypass the circuit breaker and see if the issue is resolved. If not, you likely have a bad brake controller.
And to touch on the code clearing part, there is not a way to clear codes aside from unplugging the controller and plugging back in.
Jameson C.
3/20/2025
@Phiib I'll let Andrew know he was helpful. This is a rare occasion around here. Just kidding he knows his stuff haha.
Dennis E.
10/1/2023
MY prodigy works great andei am able to adjust it for fine tune regarding weight HOWEVER OCCASIONALLY the brakes grab to sometimes skid on the first application of the pickup brake pedal. If I tap pedal lightly when I know I need brakes it doesn't grab on second depression of pedal. It's maddening and dangerous
Mike L.
10/3/2023
@DennisE Is the brake controller throwing any error codes? If not, you'll want to check the brake output circuit on the trailer. Make sure all the connections are secure. Make sure the main trailer ground connection is tight and the metal the ground wire is attached to isn't corroded. Inspect the brake assemblies themselves, make sure they're properly adjusted and that the magnet, brake shoes and drums are in good shape.
Mike L.
12/13/2023
@Ken You shouldn't be seeing the OL without a trailer connected. I'd suspect the trailer connector on the vehicle is damaged or corroded. If you can see white or green corrosion on the pins, chances are the corrosion has spread to the interior of the connector, and is causing adjacent circuits to bridge. If that is indeed your problem, replacing the 7-way trailer connector will fix your issue.
Mike L.
8/1/2023
@JohnH The 'OL' error code indicates an overload condition. There are several possible reasons for the error message you're seeing. It could be a short in one or more of your brake assembly magnets, the brake circuit wire could have rubbed the insulation bare from rubbing on the trailer frame or maybe a corroded trailer connector. Check the vehicle socket and the trailer plug for signs or corrosion or trapped foreign material. Any of these conditions could allow the brake circuit to inadvertently connect to another and that could cause the OL code. There are several potential causes for this OL error message. These include a short in one or more of your brake assemblies' magnets, the brake circuit wire rubbing on the trailer frame or even just a corroded trailer connector. Check the vehicle socket and the trailer plug for signs or corrosion or trapped foreign material. Either of these conditions could allow the brake circuit to cross over to an adjacent circuit that could cause the OL code. Check that out, and let us know what you find!
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11/4/2025
I have a Brake Evn controller that shows .c. for connected but doesn't brake using the pedal. It does brake manually.